(a) Field of the Invention
The present invention relates to a power transmission device including two or more pistons and a method of determining a return spring force in the power transmission device.
(b) Description of the Related Art
In general, a power transmission device is used to selectively transmit power of one member to another member. The power transmission device is also used to stop a rotating member.
The typical power transmission device includes a piston mounted in a housing thereof to form a piston chamber in a space between the housing and the piston, and a clutch pack configured to selectively and operatively couple the housing to a power transmission hub. The power transmission device may further include a return spring for applying an elastic force to the piston. The elastic force acts against an operating hydraulic pressure supplied to the piston chamber.
When an operating hydraulic pressure is supplied to the piston chamber, the piston is pushed in an axial direction to apply an axial force to the clutch pack. In this case, a plurality of friction disks and a plurality of separate plates included in the clutch pack are brought into contact with each other by the axial force, and are coupled to each other by frictional force. Accordingly, the power of the housing is transmitted to the power transmission hub.
When the operating hydraulic pressure supplied to the piston chamber is exhausted, the piston returns to an initial position by the action of the elastic force of the return spring. In this case, the friction disks and the separate plates included in the clutch pack are separated and disconnected from each other. Accordingly, the power of the housing is not transmitted to the power transmission hub.
Recently, research to increase the magnitude of the axial force applied to the piston while reducing the magnitude of the operating hydraulic pressure in the power transmission device has actively been in progress. Accordingly, a technique for mounting two or more pistons in one power transmission device has been undergoing development.
As the number of pistons is increased, the piston area to which the operating hydraulic pressure is applied is expanded, thereby increasing the magnitude of the axial force applied to the pistons. However, if the magnitude of the axial force applied to the pistons is increased, a return spring force that is applied against the axial force needs to be increased. Particularly, it is very important to appropriately determine the return spring force in order to properly operate the power transmission device including two or more pistons. However, the research on determination of the return spring force has not yet progressed.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.